The invention is directed to clay based drilling or completion muds containing polymer additives effective for drilling or completion operations at temperatures in the range of about 90.degree. to 200.degree. C. having increased thermal stability and reduced viscosity.
Drilling and completion muds must possess a number of sometimes contradictory physical and chemical properties, in order to perform their function properly.
Consequently, water drilling muds generally contain a number of component ingredients, including: (a) fresh water or water containing electrolytes such as sodium chloride, potassium chloride or calcium chloride, or can be seawater; (b) clay, chosen to suit the saline concentration of the water and to provide the necessary rheological properties; (c) one or more viscosity increasing polymeric materials; (d) a fluid loss additive which can be a polymeric material; (e) a dispersant to reduce the viscosity of the clay by deflocculating it; and (f) one or more weighting or density increasing agents.
The present practice to drill with muds having low viscosities wherever the ground permits, and the problems caused by thickening of the mud at great depths, places considerable importance on dispersants in the formulation of water drilling muds. It is necessary when using low viscosity muds to keep the viscosity at the required level while retaining adequate thixotropic properties and suspensoid capacity; and when using high viscosity muds to prevent or at least limit a rise in viscosity.
Polyphosphates have been used extensively. However, polyphosphates are not stable at elevated temperatures. Lignin is an effective agent, but when contaminated by certain electrolytes, particularly calcium salts, it precipitates thereby losing its effectiveness.
Ferrochrome lignosulphonates, for example Brixel.TM. NF 2 manufactured by CECA, are extremely effective up to a concentration of approximately 10 grams/liter (g/L) which is the limit of their dispersion effect; beyond that, they are used as plugging and fluid loss agents. Their action is not affected by calcium salts. However, they have only medium heat stability, which is a serious drawback for deep drilling operations. In addition, discharge of chromium salts which are toxic, is increasingly controlled for ecological reasons, and use of these complex lignosulphonates will probably be banned in the future.
Other dispersants comprise acrylic acid salts (as described in U.S. Pat. No. 3,764,530), which are not effective in the presence of electrolytes, and maleic anhydride and styrene sulphonate copolymers (as described in U.S. Pat. No. 3,730,900).
All dispersants do not function in the same way. However, it has been demonstrated for certain dispersants, such as ferrochrome lignosulphonate (in the article SPE 8225), and it seems likely for all such agents, that adsorption on laminated particles, on the faces or edges of such particles depending on the electron donor or acceptor functions in the dispersant, prevents the clay from flocculating or gelling, by preventing edge-to-face bonds common in clay suspensions, to an increase in the forces of repulsion and/or a reduction in the forces of attraction between particles.
Reduction in dispersion capacity at high temperature may have at least two causes, apart from those connected directly with drilling conditions: (a) chemical deterioration of the dispersant; and (b) desorption of the dispersant at the elevated temperature.